So some dead schmoes get recruited to fight invisible aliens IN SECRET. And they fight aliens of increasing difficulty on a 1-100 point scale. And after X number of years this manga has been running, they're still no closer to finding the true nature of the Gantz/One Piece/Nine-tailed Fox.

Sorry, but it actually does sound a lot like Naruto, DBZ, and the like.

Plus, if dead people can be revived to fight the aliens, why can't they be revived if they're killed while fighting the aliens?